Green approach in metal nanoparticle-embedded antimicrobial coatings from vegetable oils and oil-based materials

ABSTRACT

The present invention generally relates to a method of making nanoparticles and uses thereof. In particular, the invention relates to methods of making metal nanoparticles (MNPs). The invention also relates to antimicrobial uses of the nanoparticles.

This application claims the benefit of priority to U.S. ProvisionalPatent Application No. 61/011,214 filed Jan. 15, 2008 and to U.S.Provisional Patent Application No. 61/125,782 filed Apr. 28, 2008, bothof which are hereby incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention generally relates to a method of makingnanoparticles and uses thereof. In particular, the invention relates tomethods of making metal nanoparticles (MNPs). The invention also relatesto antimicrobial uses of the nanoparticles.

BACKGROUND

Renewable resources, such as plants and crops, are inexhaustible andclean sources of materials that, when used in industrial processes,often produce by-products. Strategic utilization of such industrialby-products (i.e., biomonomers) as starting materials for generatingvalue-added products and building blocks in chemistry will have broadimpact in industrial economy as well as in sustainable development(Lichtenthaler 2002, Corma 2007, Goldemberg 2007). The efficientutilization of renewable resources is possible for developing novelmonomers, polymers, chemicals, and soft nanomaterials (John, Soft Matter2006, Vemula, J. Am. Chem. Soc. 2006, Vemula 2007, John 2001, John 2002,John 2004, John, Angew. Chem. Int Ed. 2006, Rostrup-Nielsen 2005,Pagliaro 2007, Biermann 2000). Polymers are among the most importantproducts of the chemical industry and are used for versatileapplications in everyday life. Employing agricultural/industrialby-products in polymer applications (for instance, the production ofpackaging, textiles and other functional materials) will be highlyadvantageous due to their properties of being renewable andbiodegradable. Additionally, these biomonomers may be converted intovaluable polymers or novel amphiphiles to produce soft nanomaterials.

A primary by-product of cashew nuts is cashew nut shell liquid (CNSL),which is extracted from the by-product shells of the cashew nut. One cansynthesize free radically polymerizable monomers from cardanol (acompound derived from CNSL) by simple modifications and then polymerizethem for use in coating applications (John 1992, John 1993).

In addition to CNSL oil, another polymerizable oil is vegetable oil.Common household oil paint, the oldest form of modern paints, uses abinder that is derived from vegetable oils obtained from linseed or soyabean. Alkyd paints are based on alkyd resins (vegetable-derived dryingoils), which contain a variety of polyunsaturated fatty-acid chains,commonly linoleic and linolenic acid and their triglycerides (Daniel1964, Metzger 2006, Bieleman 2000), which undergo free radical-mediatedautoxidation during the curing/drying process (Black 1978, Reich 1969)(FIGS. 1 a-c). The use of naturally generated free radicals enables oneto generate valuable oil-based products.

Coatings can be used to decorate or protect surfaces of interest(Bohannon 2005, Crisp 2003, Klaus 1999). In general, several naturaloils, drying oils in particular, are excellent coating materials, andwhen exposed to air, they form a tough scratch-free film as a result ofthe oxidative drying (lipid autoxidation) process that occurs through awidely accepted ‘free radical’ mechanism in the presence of atmosphericoxygen (Black 1978, Reich 1969) (FIG. 1 c). In addition, literaturereports suggest that free radicals are known to reduce metal salts totheir uncharged MNPs (Zhang 2006, Okitsu 1997). Free radical-induced MNPsynthesis is well studied (Zhang 2006, Okitsu 1997).

Several methods have been reported for the preparation oforganic-inorganic hybrid materials; and most of the techniques used toincorporate metals into polymeric matrices involve either chemicalreactions such as reduction (Aymonier 2002), mixing preformed metalnanoparticles with polymers (Liu 2005), or complicated physicaltechniques (Heilmann 2002), such as sputtering (Dowling 2003), plasmadeposition (Jiang 2004), and layer-by-layer deposition (Dai 2002). Allof these techniques add time, cost, multistep synthesis, and complexityto the overall process of fabricating metal-particle-doped materials.

Metal nanoparticles have attracted a great deal of attention because oftheir unusual optical and electronic properties (Colvin 1994) withpotential application in the area of catalysis, (Hoffman 1992) electronmicroscopy markers, (Baschong 1990) gene therapy (Elghanian 1997) andsensors. (Shipway 1999) Recent interests focused towards developing newapplications of nanoparticles having antifungal, antibacterialproperties and can be used as coating materials or packaging materials.Attempts have been made to design such materials by embedding aantimicrobial agent in existing well known coating or packagingmaterials. Silver nanoparticles are known for its antibacterialproperties have been used in fabrics, polymer for various applications.(Prashant 2005, Wang 1994, Chou 2005) Prashant et al. attached thesilver nanoparticles on the surface of polyurethane foam and used it forwater filter to avoid the bacterial contamination of surface water.(Prashant 2005) Wang et al prepared the antibacterial utltrathin film oftitanium phosphate containing silver nanoparticles. (Wang 1994)Antibacterial cellulose acetate has also been made by incorporatingsilver nanoparticles in cellulose acetate based membrane. (Chou 2005)

Silver nanoparticles have also been used to incorporate coatingmaterials to make antibacterial paints. In most of the approaches,either nanoparticles were synthesized separately and attached todifferent support, or silver ions were reduced in the presence ofsupport using external reducing agent. Perhaps the same process can beused for the synthesis of nanoparticles and integrating them in coatingmaterials for different application. Recently Willner and co-workersformed super lattice of citrate stabilized gold nanoparticles andcyclobis(paraquat p-phenylene) on the ammonium-functionalized indiumtinoxide (ITO) surface using electrostatic interaction. (Shipway 1999)The Au nanoparticles in the super lattice provide a rough conductivearray for the electrochemical sensing of the π-donor aromatic compound.Mirkin and co-workers used the optical properties of gold nanoparticlesfor the detection of DNA down to a concentration of 50 fM. (Taton 2000)Gold show catalytic activity for the oxidation of carbon monoxide atnanoscale at higher temperature (Haruta 1988). This catalytic activityis due to high surface free energy of nanoparticles, which makes themuseful for protective gas masks and household room air fresheners etc.Gold particles have also been recognized as good catalyst for water gasshift reaction, propylene epoxidation, and benzene oxidation etc. (Bond1999).

Much of the recent research focused on developing metalnanoparticles-based flat panel displays, radio frequency identificationtags, sensors and other disposable electronics. Future technologydemands the organic substrate based devices which can be fabricatedentirely by printing to reduce the costs associated with lithography,vacuum processing and ultra clean room conditions. The main challenge isto use the low temperature conductor suitable for printing and inkjetprinting technology compatible to fabricate at low temperature on lowcost plastics. Metal nanoparticles have also been investigated for theelectronic applications because of possibility of their use in printingcircuits on plastic. (Huang 2003) The low resistance circuits werefabricated on plastic using alkanethiol protected metal nanoparticlesdispersion as an ink at lower temperature.

Other than electronic application, the nanoparticles have also been usedas a pigment in paints due to surface plasmon resonance in the visibleregion. More precisely, gold and silver nanoparticles have been known asan artistic ruby and yellow colorant for stained glass and fineglassware, due to their inherent surface plasmon absorption. The rubyred or yellow color of the stained glass is stable for hundreds ofyears. In contrary, the red color of organic dyes in traditional paintsoften fades away within several years due to the short lifetime oftypical organic compounds. Nippon Paint has recently developed thetechnology for the use of paints for cars, based on a polymer-stabilizedgold colloid. This paint appears black in shaded areas and red inilluminated areas, giving a dynamic effect as the vehicle is in motiondue to varying light conditions. (Iwakoshi 2003) Use of this type ofdynamic color effect could be envisaged for use in security devices,such as ‘watermarking’ of valuable or confidential documents, andbiomedical testing kits. Titanium dioxide nanoparticles have been usedin paints as a whitener as well as photo-active catalyst for hygiene orself cleaning application.

Another advantage of using metal nanoparticles in paints is their highreflectivity of infrared radiation. The heat-loss occurs in three ways:convection, conduction and radiation. Insulation is quite effective toreduce the heat loss due to convection and conduction however it havevery little effect on heat loss due to radiation. Metal nanoparticles(Ag, Au, Al, Cu, Rh) have reflectivity of 98-99% in the infrared (IR)portion of the spectrum so paints containing metal nanoparticles willincrease the reflection of radiant heat. Therefore use of such kind ofpaints inside of exterior will reduce heat loss by radiation. Currentpaints manufactured by ChemRex are claimed to reflect 30% of the radiantincident heat. The radiation of a room at 70° F. will peak at awavelength of 10 microns, according to the black-body equation.Calculation, based on refractive indices of the particles and the paint,and a wavelength of 10 μm and particles size, shows that thereflectivity (scattering) increases linearly with the particle numberdensity but not with the particle size.

Optical behavior of nanoparticles can be tuned by tailoring the shape ofthe particles. For example the optical absorption of gold nanoparticlescan be tuned from visible region to near-infrared (NIR) region ofelectromagnetic spectrum by tailoring the shape of the particles fromspherical to rod or triangular shape. The NIR absorption of the goldnanotriangles is expected to be having applications in hyperthermia ofcancer cells and in IR-absorbing optical coatings. Sastry and co-workesshowed that triangular gold nanoparticles coated glass films are highlyefficient in absorbing IR radiation for potential architecturalapplications where the temperature in a compartment need to control dueto expose of an infrared radiation. (Shankar 2005)

Most of the methods demand the synthesis of metal nanoparticles at largescale. Therefore it is most important to have a protocol for thesynthesis of metal nanoparticles dispersion at large scale with precisecontrol over the particles size and high metal concentration, yet mostimportantly keeping low production cost. Preparation of monolayerprotected gold nanoparticles was achieved using the method previouslydocumented by Brust et al. (Brust 1994) The key requirement for thescale-up of the nanoparticles was to reduce solvent levels used duringthe preparation stages. For example, it was reported that to prepare ca0.25 g of the thiol-stabilized nanoparticle according to the Brustmethod would require ca 80 mL of toluene and 800 mL of ethanol forprecipitation and purification. By extrapolation, 3 kg of product wasreported to require 960 L of toluene and 10,000 L of ethanol. This wasconsidered impractical for commercial production. With the describedmodifications, 0.5-1 kg quantities of gold chloride could be used toproduce nanoparticles in 20 liter reaction vessels that were consistentin gold assay of the final product and also analytically similar foreach batch. (Bishop 2002) The success of this technology is thought tobe due to the low mobility of these nanoparticles during the earlystages of heat treatment (50-150° C.) and also to their tendency toself-assemble and form ‘loose’ gold films before thermal decompositionof the stabilizing thiol ligand occurs.

Drying oils/alkyd resins are known as one of the oldest and the cheapestcoatings materials and have attracted renewed interest because they arefrom renewable resources, like plant oils and independent of limitedsupply of petroleum-based products. Alkyd emulsions and high solid alkydresins have shown a lot of success fulfilling the environmental demands.Moreover, life-cycle analysis of alkyd emulsion paints showed lesseffect on the environment than those based on acrylic dispersions. Thepossibility to obtain versatile, low cost, renewable, and low VOCemission products makes alkyd paints very attractive materials.

Silver and silver-based compounds are highly antimicrobial by virtue oftheir antiseptic properties to several kinds of bacterium, includingEscherichia coli and Staphylococcus aureus (Sambhy 2006, Lansdown 2002,Kenawy 2007). Silver-based antimicrobial agents receive much attentionbecause of the low toxicity of the active Ag ion to human cells(Williams 1989, Berger 1976), as well as it being a long-lasting biocidewith high thermal stability and low volatility. However, althoughprevious studies on silver and AgNPs have revealed some insights intothe application of silver in several areas, little is known about thetoxicity of AgNPs, where the size and surface area are recognized asimportant determinants for toxicity. AgNPs have been shown to possessgood biocompatibility with mouse fibroblasts and human osteoblasts (Alt2004), and their use for biological applications has been documented aswell (Podsiadlo 2005). AgNPs are known to exhibit antibacterialproperties and various research groups have investigated the mechanismof AgNP-mediated antibacterial activity (Morenes 2005, Gogoi 2006). Asthe size of the silver particles decreases down to the nanoscale regime,their antibacterial efficacy increases because of their larger totalsurface area per unit volume (Morenes 2005, Gogoi 2006).

One important aspect to consider is that although efficientantibacterial agents have been developed (Haldar 2006, Lewis 2005), theyoften fail to reach commercial needs owing to their complex, multi-steppreparation methods and the high cost of production (Bohannon 2005). Ifthe aim is to develop a general, simple (for example, single-step)procedure to make a solid surface bactericidal, then covalent attachmentof polymers is probably not a viable option given the paucity ofderivatization-amenable functional groups on most common surfaces.

Typically, nanoparticle synthesis involves external reducing agents andtoxic organic solvents, which pose potential environmental andbiological risks. Except for a few reports (Naik 2002, Raveendran 2003),it is difficult to find fully environmentally friendly methods for MNPsynthesis.

Polymer-stabilized MNP composites (Morones 2007, Abyaneh 2007) are knownto exhibit enhanced physicochemical stability, electrical andoptoelectronic properties (Daniel 2004, Shan 2005). These composites areprepared either by simple entrapment of gold and silver nanoparticles(AuNPs and AgNPs, respectively) in a pre-synthesized polymer. Typically,the polymers have a thiol or a thiolate end group and are allowed toself-assemble on the MNPs' surface. The self-assembly occurs as a resultof the specific interaction of the sulfur end-group with the surface(Hotchkiss 2007, Liu 2007, Fustin 2006). Another approach to creation ofMNPs involves the reduction of gold salts with sodium borohydride in thepresence of thio (Zheng 2004, Shimmin 2004) or dithioester (Shan 2003)end functionalized polymers. The reaction yields hybrids with AuNPswithin the polymer shell. Physical entrapment of MNPs, however, hasobstacles. For example, physical entrapment often produces heterogeneoushybrid materials. Importantly, it requires separate synthesis andpurification of NPs and external doping into polymers (a multi-stepprocess).

Oxidative drying of polyunsaturated oils is well known. In general,several natural oils, drying oils in particular, are excellent coatingmaterials, and when exposed to air, they form a tough scratch-free filmas a result of the oxidative drying (lipid autoxidation) process thatoccurs through a widely accepted free radical mechanism in the presenceof atmospheric oxygen.

The three main steps in the preparation of MNPs involve the choice ofthe solvent medium used for the synthesis, the selection of anenvironmentally benign reducing agent, and the selection of a non-toxicmaterial for the stabilization of the MNPs (see Anastas 1998). Althoughthere are several known reducing agents, the majority of processesreported so far use reducing agents such as sodium borohydride (NaBH₄)and hydrazine (NH₂—NH₂). All of these are highly reactive chemicals andraise potential environmental and biological risks. Another and perhapsthe most important issue is the choice of a capping agent to protect andpassivate the nanoparticle surface, for better dispersion of MNPs.

Previously, novel organic-inorganic hybrid nanomaterials were preparedusing self-assembled hydro/organogels (Vemula 2007, Vemula, Chem.Commun. 2006) and LCs as media for in situ synthesis of various MNPs(Zhang 2006, Okitsu 1997, Okitsu 1996).

In a prior art process, silver nanoparticles have beendispersed/incorporated in silicon rubber to achieve an antimicrobialeffect, but in an amount less than cytotoxic silver concentration (U.S.Pat. No. 6,822,034). Silicon rubber is used in applications whichinclude, for example, pan grips, camera eye caps, handles of bicycles,slipping preventative for spectacles, various rubber sheets and rubbercoated cloth such as sheets and curtains that are used, for example inhospitals.

In another process, silver nanoparticles in organic matrix have alsobeen used for antimicrobial activity for body care products (U.S. Pat.No. 6,720,006). A suspension containing silver nanoparticles with anindividual size range of 5 to 50 nm was produced through thermalevaporation of silver into a liquid silicone oil base. Polypropylenegranules are then co-extruded with this silicone oil using a Werner &Pfleiderer equipment to produce polypropylene granules containing up to5% of the silver containing silicone oil. This master material was madeinto top sheets for diapers containing approximately 1000 ppm silver.The ELISA measurements demonstrated antibacterial efficacy.

Synthesis of nanoparticles (U.S. Pat. No. 6,974,493 and U.S. Pat. No.6,929,675) in nonpolar medium is available. Harutyunyan, et al. (U.S.Pat. No. 6,974,493) synthesized the metal nanoparticles by heating orrefluxing a mixture of two or more metal salts, such as metal acetates,and a passivating solvent, such as glycol ether, at a temperature abovethe melting point of the metal salts for an effective amount of time.Bunge, et al. (U.S. Pat. No. 6,929,675) followed different strategywhich involves the thermal decomposition of organomettalic complexes ofmetal in organic phase. In this method, a solution of (CU(C₆H₂(CH₃)₃)₅,(Ag(C₆H₂(CH₃)₃)₄, or (Au(C₆H₂(CH₃)₃)₅ is dissolved in a coordinatingsolvent, such as a primary, secondary, or tertiary amine; primary,secondary, or tertiary phosphine, or alkyl thiol, to produce a mesitylprecursor solution. This solution was decomposed by injecting it into anorganic solvent heated to a temperature of approximately 100° C.

In yet another process, organically functionalized metal nanoparticleshave been synthesized by mixing a metal precursor with an organicsurface passivant and reacting the resulting mixture with a reducingagent to generate a free metal while binding the passivant to thesurface of the free metal to produce organically functionalized metalparticles (U.S. Pat. No. 6,103,868).

There is a need for a simpler, environmentally friendly process ofpreparing MNP-embedded materials. Accordingly, an objective of thepresent invention is the preparation of potent antibacterial coatings ina single step at ambient conditions without using external reagents orexcessive energy for practical applications. Capitalizing on theversatility and reliability of oils (such as oil-based paints), thepresent invention uses an oxidative drying mechanism (lipidautoxidation) in the presence of metal salts to generate and stabilizeMNPs (e.g., AgNPs) in oil, which competes, e.g., with previouslyimplemented AgNP-based bactericidal agents (Morones 2005, Gogoi 2006).The process of the present invention thus provides an environmentallyfriendly method for making antimicrobial coatings containing metalnanoparticles.

SUMMARY OF THE INVENTION

The present invention relates to a successful, environmentally friendlyprocess for synthesizing antimicrobial metal MNP-embedded materials,which can be performed in a single step. The naturally occurringoxidative drying process in oils, involving free radical exchange, isused as the fundamental mechanism for reducing metal salts anddispersing MNPs in an oil media (e.g., cashew nut shell liquid (CNSL) orvegetable oils), without the use of any external reducing or stabilizingagents. The well-dispersed MNP-in-oil dispersions can be used directlyon nearly all kinds of surfaces such as wood, glass, steel, and variouspolymers. For example, surfaces coated with silver nanoparticle-in-oildispersions prepared according to the present invention exhibitexcellent antimicrobial properties. The present invention takesadvantage of free radicals created during an oxidative drying process ofoils to reduce metal salts, thereby creating a dispersion of metalnanoparticles throughout the oils.

The present invention is particularly useful for preparing antimicrobialcoatings, decorative coatings, and antibacterial coatings in commonplaces like hospitals, public places, restaurants, etc. The process ofthe present invention may also be used to prepare antimicrobial topicaloils and antimicrobial soaps (for hand washing or general washing), andmaterials useful in linoleum floorings, building materials, glasscoatings for UV/IR reductions, and antistatic coatings.

In particular, the nanoparticles of the invention are suitable for usein coating materials for hospital countertops, beds, and general medicalequipment. The nanomaterials are also suitable for incorporation intoflooring materials, such as vinyl flooring, linoleum flooring, etc. Thenanoparticles of the invention have especially good antimicrobialactivity against Methicillin-resistant Staphyloccus Aureus (MRSA)bacteria (also known as a super-bug), which are a serious problem in thehealthcare industry. In general, antimicrobial compositions of thepresent invention would be beneficial when used in any public placeswhere it is desirable to prevent MRSA infections.

The preparation of MNPs without using external reagents and in a singlestep (e.g., in situ) by excluding extra purification processes ortransfer protocols has significant advantages over current methods. Toovercome the above-mentioned hurdles, the present invention usesefficient supramolecular organic soft materials as hosts for thesynthesis and stabilization of inorganic MNPs (Mallia 2007, Vemula 2007,Vemula, Chem. Commun. 2006). The present invention also includes the useof a naturally occurring autoxidation/drying process in vegetable-baseddrying oils as a tool to prepare MNPs.

In one embodiment, the present invention relates to a method ofpreparing metal nanoparticle-embedded antimicrobial coatings from CNSL(i.e., cardanol, alkyd resins, urushiol, or other polyunsaturatedoils/acids) using either naturally occurring or catalytic autoxidationor an oxidative drying process. Additionally, this method takesadvantage of free radicals generated during the process of drying oilsand oil-based materials (e.g., drying oils/alkyd paints) to reduce metalsalts and create a dispersion of MNPs in an oil or oil-basedmaterial—e.g., silver- and gold-nanoparticle (AgNP- and AuNP-) embeddedpaints (in situ). AgNP-embedded oils (e.g., vegetable oils) andoil-based materials are particularly preferred due to their potentialbactericidal activity.

In another embodiment, the invention relates to the synthesis andstabilization of AuNPs and AgNPs in a bio-based cardanyl acrylatepolymer. During the drying process, naturally occurring cross-linking(autoxidation) of unsaturated alkyl chains is used as a tool to reducemetal salts and bind the nanoparticles. Since the nanoparticles aregenerated in situ, the use of external hazardous reducing agents isavoided.

In another embodiment, the present invention relates to a process fordirect synthesis of nanoparticles into drying oil/alkyds/alkyd modifiedresins. Drying oil is a kind of vegetable oil, which dries at ambientconditions to form glossy films, and has been practiced for centuries inoil paintings, art materials and alkyd resins and coatings. Thenanoparticles were formed by dissolving a salt in an oil medium andshaking. The stability and shelf life of nanoparticles is comparable tonanoparticles synthesized using a conventional process because of thepassivation with polymer formed during the reaction. Nanoparticlesexhibit prominent features in the UV-visible region of theelectromagnetic spectrum due to the electronic transitions. Thisdispersion is useful for various applications such as novel chemicalreactions on nanoscale curved surface and self-assembly of surfacemodified nanoparticles. Metal nanoparticle dispersions could also findapplications in conducting coatings, fluorescent dispersions, antistaticand antimicrobial coatings. These metal nanoparticle-containingvegetable oils can be used as a colored coating on various substrate.More precisely, gold nanoparticles can be used as an artistic rubycolorant for stained glass and fine glassware, due to their inherentsurface plasmon absorption. Hence, the present invention may be used forgraceful, yet stable, colored glass, ceramics or any surfaces by usinggold nanoparticles doped paints as thin films on these materials. Thereare a few hurdles that are avoided by using the process of the presentinvention. For instance, because the process of the present inventiondoes not use an external reducing agent and stabilizing agent, furtherpurification processing is not necessary; and coagulation of goldnanoparticles at higher concentrations is avoided as well.

The present invention relates to an antimicrobial composition that has ahomogenous mixture of a drying oil and metallic nanoparticles with aparticle size of 1 to 50 nm, where the composition is effective as anantimicrobial. Suitable drying oils include, for example, cashew nutshell liquid, linoleic acid, poppy oil, soyabean oil, urushi oil,linseed oil, sunflower oil, tung oil, alkyd resins, other vegetableoils, and combinations thereof. Suitable metallic nanoparticles include,for example, those with a metal such as silver, gold, nickel, platinum,palladium, cadmium, zinc, copper, and combinations thereof.

The metal nanoparticles are present in an amount that is antimicrobiallyeffective, but less than a cytotoxic silver concentration. Thenanoparticles are preferably present in an amount ranging from more than1 nmol/L to less than 1 μmol/L. In the present invention, the metallicnanoparticles are dispersed in the drying oil and are present in anamount of 1 to 2,000 ppm, more preferably 5 to 1,000 ppm, and mostpreferably 10 to 250 ppm.

The present invention also relates to a method for preparing metalnanoparticles in a drying oil, comprising the steps of: (a) mixing asolution comprising metal ions with a solution comprising a drying oilin the presence of an organic solvent or an organometallic compound; (b)agitating the mixture for a period of 12 to 24 hours; and (c)polymerizing the drying oil by autoxidation to form metal nanoparticlesin a polymerized oil.

The metal nanoparticles may be hydrophobic. Suitable metal ions include,for example, gold, silver, nickel, platinum, palladium, cadmium, zinc,copper, and combinations thereof. Suitable drying oils include, forexample, cashew nut shell liquid, linoleic acid, poppy oil, soyabeanoil, urushi oil, linseed oil, sunflower oil, tung oil, alkyd resins, andcombinations thereof. Suitable organic solvents include, for example,n-hexane, chloroform, heptane, octane, petroleum ether, benzene,toluene, turpentine, and combinations thereof. Suitable organometalliccompounds include, for example, silver benzoate, metal acetylacetonates, metal carbonyls, nonpolar metal salts, iron acetylacetonate, platinum acetyl acetonate, nickel acetyl acetonate, cobaltacetyl acetonate, cobalt acetate, iron petacarbonyl, and combinationsthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. (1 a): Chemical structures and synthetic scheme of poly(cardanylacrylate) (PCA). (1 b) general mechanism for free radical mediatedcross-linking of cardanyl polymer side chains.

FIG. 2. (2 a) Images of glass slides coated with PCA containing AuNPs(left), AgNPs (middle) and un-reduced metal salts (right); and (2 b)absorption spectra of PCA after addition of chloroauric acid (i), andafter AuNPs formed (ii). Similarly, after addition of silver benzoate(iii) and after AgNPs formed (iv).

FIG. 3. SEM and TEM images of PCA. (3 a) SEM images of PCA with AuNPs;(3 b) SEM images of PCA with AgNPs; (3 c) TEM images of PCA with AuNPs;and (3 d) TEM images of PCA with AgNPs.

FIG. 4. Chemical structures of poly(pentadecylphenyl)acrylate (PPDA),which is a saturated analogue of PCA.

FIG. 5. Chemical structures of common fatty acids in drying oils, andsynthesis and characterization of AgNPs in alkyd resins; (5 a) chemicalstructures of fatty acids with different degrees of unsaturation thatare present in alkyd resins; (5 b) structures of general triglyceridespresent in alkyd resins; (5 c) general mechanism for the freeradical-mediated autoxidation process in drying oils; (5 d) schematicdiagram of in situ synthesis and stabilization of MNPs in drying oils;(5 e) transmission electron micrograph of AgNPs synthesized in dryingoils with an average size of 12-16 nm. The inset shows the absorptionspectra of AgNPs with a surface plasmon resonance band; spectra wererecorded at (1) 5 min and (2) 24 h after the addition of silver benzoateto the oils; and (5 f) kinetics of the metal salt reduction process,time required for nanoparticle synthesis is plotted; the addition of acatalyst (Fe²⁺) enhanced the generation of free radicals, whichincreased the rate of nanoparticle synthesis; in contrast, the additionof DMSO, which is a well-known free radical scavenger, completelyprevented nanoparticle synthesis.

FIG. 6. Images of metal-salt-containing drying oils, andnanoparticle-embedded paint coatings. (6 a) images of plain commerciallyavailable drying oil, and silver benzoate and chloroauric acid dissolvedin drying oils (left to right); (6 b) images of paint coatings withoutnanoparticles (left panels), AgNPs (middle panels) and AuNPs (rightpanels) on glass (b) surfaces; and (6 c) images of paint coatingswithout nanoparticles (left panels), AgNPs (middle panels) and AuNPs(right panels) on polymer surfaces.

FIG. 7. AuNP synthesis in cardanol-based polymer films. (7 a) synthesisof cardanyl acrylate and its polymerization to form PCA with a mixtureof unsaturated alkyl chains, which was used for synthesis of AuNPs (theright image shows the AuNP-embedded polymer film); and (7 b) synthesisof pentadecylphenyl acrylate (a saturated analogue), and itspolymerization to produce a sticky transparent film that failed to showAuNP synthesis owing to the absence of the autoxidation process (theright image shows the sticky clear polymer film).

FIG. 8. Summary of the antibacterial properties of AgNP-containingpaints. (8 a) and (8 b): photographs of commercially available blankglass slides without coating (i), glass slides coated with onlydrying-oil paint without nanoparticles (ii) and glass slides coated withAgNP-containing drying-oil paint (AgNP-embedded paint) (iii), onto whichaqueous suspensions of approximately 5×10⁶ cells ml⁻¹ of S. aureus cells(8 a) and 5×10⁷ cells ml⁻¹ of E. coli cells (8 b) in PBS solution weresprayed, followed by drying in air for 5 min, covering with solid growthagar and incubating at 37° C. overnight. Each black dot corresponds to abacterial colony grown from a single surviving bacterial cell.

FIG. 9. (9 a) UV-Visible spectra of silver benzoate in oil as a functionof time. (9 b) UV-visible spectra of Ag-oil (curve 2) and pure oil(curve 1) on the glass substrate.

FIG. 10. Photograph of pure oil coated glass (10 a), Ag-oil coated glass(10 b), Polycarbonate (10 c), Polymethylmethacrylate (10 d).

FIG. 11. (11 a) TEM image of Ag-oil film on carbon coated copper gridfilm formed by solvent evaporation technique. (11 b) Histogram ofparticles size measured from (11 a).

FIG. 12. NP-paint interaction with bacteria. Schematic representation ofAgNPs-paint coated on desired surface which decontaminate the bacteriawhen it get in contact to the bacteria.

FIG. 13. Schematic diagram for the synthesis of gold nanoparticles usingvegetable oil.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process for an environmentallyfriendly process for preparing antimicrobial MNP-embedded materials in asingle step, which utilizes an oxidative drying step in the presence ofa metal salt. The naturally occurring oxidative drying process in oils,involving free radical exchange, is used as the fundamental mechanismfor reducing metal salts and dispersing MNPs in the oil media, withoutthe use of any external reducing or stabilizing agents. Acrylates andpolyurethanes derived from CNSL, for example, can be used as startingmaterials for oxidative drying induced nanoparticle formation accordingto the present invention.

In the process of the present invention, no solvents are required forthe synthesis of MNPs. Instead, the commercially availableenvironmentally benign drying oils are used. Additionally, regarding thereducing agent, free radicals naturally generated in situ during thedrying process of the present invention are used as reducing agents.This process does not require heating, and moreover the system is mild,renewable, cheap and non-toxic in nature. Regarding the choice of acapping agent to protect and passivate the nanoparticle surface forbetter dispersion of MNPs, the present invention uses the polymer (e.g.,alkyd) resin itself acts as the protecting agent. Fatty acids and insitu-generated aldehydes and other intermediates act as stabilizingagents for MNPs.

The presence of several in situ-generated free radicals such as LOO.,LO. and L. (L=lipid chain) during autoxidation of drying oils is usefulfor the reduction of metal salts to synthesize MNPs in situ (FIG. 5 c).For example, when silver benzoate was used as the precursor for MNPs indrying oil and the conventional ambient drying process was used, visualchanges were observed. The oil phase became light yellow in color withtime, which indicated the formation of AgNPs.

The process of the present invention was also tested with other metalsalts—e.g., chloroauric acid (HAuCl₄) for an AuNP preparation.Appropriate choice of the organometallic salts facilitates thesolubility of nanoparticle precursors into the oil medium. Silver salts,for example, undergo ligand exchange with fatty acids, causing the metalions to dissolve in the oil and subsequent reduction by the freeradicals to form nanoparticles (Zhang 2006, Okitsu 1997) (FIG. 5 d).

Silver benzoate may be used as a starting material for preparing AgNPs,and chloroauric acid (HAuCl₄) may be used as a starting material forpreparing for AuNPs to synthesize MNP-embedded polymers. The presence ofseveral in situ generated free radicals, such as LOO., LO., and L.(L=cardanyl lipid chain), during the autoxidation of PCA could be wellutilized for the reduction of metal salts to synthesize MNPs in situ.

In another embodiment, the present invention involves a process for thepreparation of coating materials containing nanoparticles by a simpleshaking process. This process involves dissolution of organometalliccomplex in oil medium and then in situ formation of nanoparticles in theorganic phase due to drying of oil into polymeric matrix. The criticalgoal of the present invention is to provide an improved simple processfor the nanoparticles based coating/flooring materials withantimicrobial (e.g., antibacterial) activity.

As used herein, the term “antimicrobial” refers to a product that iscapable of destroying or inhibiting the growth of one or moremicroorganisms, including bacteria, protozoa, and viruses, preferably toan undetectable level.

This nanoparticles synthesis process was also simplified and silvercompounds (e.g., silver nitrate, silver benzoate, etc.) were used as aprecursor for the synthesis of silver nanoparticles directly from oil.In this process, the nanoparticle precursor was dissolved directly inoil phase, which upon reduction during drying process of oil generatesnanoparticles. FIG. 9 is the UV-visible spectra of solution of silversalts in oil phase as a function of time.

One embodiment of the present invention relates to the use of a dryingoil which mediates the formation of nanoparticles. This could be anykind of vegetable drying oils including alkyd resins and alkyd modifiedresins and paints. For example, linoleic acid, poppy oil, tung oil,other plant exudates such as urushi and cashew nutshell liquid may beused

In another embodiment, the present invention is not limited to metalsalts, and any other kind of organomaterial compound may be used, suchas the class of metal acetyl acetonates (e.g., copper acetyl acetonate,iron actyl acetonate nickel acetyl acetonate, copper acety acetonate),class of metal carbonyls (e.g., iron pentacarbonyl, platinum acetylacetonate, silver benzoate, silver nitrate, etc.

Another important embodiment of the present invention is this processcan be performed using various organic solvents with differentpolarities such as n-hexane, chloroform, heptane, octane, petroleumether, benzene, toluene and turpentine. Especially this is a key resultwhen it comes to the commercial use of the nanoparticles, this processwould help us to generate and store metal nanoparticles in severalorganic solvents and can be fulfilled the needs of the customers whooften demands supply of metal nanoparticles in various solvents forspecific applications and requirements.

EXAMPLES

The present invention is next described by means of the followingexamples. The use of these and other examples anywhere in thespecification is illustrative only, and in no way limits the scope andmeaning of the invention or of any exemplified form. Likewise, theinvention is not limited to any particular preferred embodimentsdescribed herein. Indeed, modifications and variations of the inventionmay be apparent to those skilled in the art upon reading thisspecification, and can be made without departing from its spirit andscope. The invention is therefore to be limited only by the terms of theclaims, along with the full scope of equivalents to which the claims areentitled.

Example 1

To investigate nanoparticle synthesis on surfaces, various surfaces suchas glass, polypropylene, and poly(methyl methacrylate) were coated withmetal ion-containing drying oils (e.g., oil-based paints) (FIG. 6).After about 6 hours of drying at ambient conditions, gold paint turnedpink in color and silver paints turned slightly brownish yellow,indicating the formation of AuNPs and AgNPs in the coatings,respectively. It is likely that free radicals generated in situ duringthe autoxidation are responsible for the reduction of metal salts togenerate nanoparticles.

The presence of AgNPs and AuNPs was confirmed by spectroscopic(ultraviolet-visible) and transmission microscopic techniques. Thestability and shelf life of nanoparticles synthesized in drying oils arecomparable or even better than those of nanoparticles synthesized usingconventional processes (for example, sodium borohydride, citric acid andso on). The higher stability of the nanoparticles is due to thestabilization of the nanoparticles by the polymer matrix formed duringthe autoxidation. The stability of the nanoparticle film was confirmedby heating the nanoparticle-oil film at ambient conditions and was quitestable up to 200° C. for an hour without significant aggregation.

The preparation of a synthetic polymer system and exploration of themetal salt reduction to generate MNPs within the polymer gives furtherinsight into the mechanism of autoxidation of unsaturated alkyl chainsthat produce free radicals to reduce metal salts. Cardanol (obtainedfrom thermal treatment of CNSL) exists as a mixture of four componentsdiffering in the degree of unsaturation in the side chain: 5% of3-(pentadecyl)-phenol, 49% of 3-(8Z-pentadecenyl)-phenol, 17% of3-(8Z,11Z-pentadecadienyl)-phenol, and 29% of3-(8Z,11Z,14Z-pentadecatrienyl)-phenol (Tyman 1979) (FIG. 1 a). Cardanylacrylate (CA) was synthesized by procedures reported earlier (John1992). Subsequently, solution polymerization was achieved usingazobisisobutyronitrile (AIBN) in toluene to obtain PCA, as shown in FIG.1 a. The PCA was dissolved in chloroform and could easily be cast into athin transparent and sticky film by a solution-casting technique.

The resulting polymer has an acrylic backbone with many unsaturatedalkyl side chains, which are easily amenable to the oxidative dryingprocess, similar to the conventional drying oils (John 1992, John 1993).In previous studies, the oxidative drying (lipid autoxidation) processof poly(cardanyl acrylate) into crosslinked networks was demonstrated byvarious techniques (John 1992, John 1993). To prove theautoxidation-mediated metal salt reduction, poly(cardanyl acrylate) wasdried in the presence of HAuCl₄, which produced a AuNP-embeddedcrosslinked polymer that was coated on a glass slide (FIG. 7 a). As acontrol experiment, a polymer was synthesized with a saturatedhydrocarbon chain, poly(pentadecylphenyl acrylate) (FIG. 7 b). Thesaturated analogue failed to undergo oxidative drying (lipidautoxidation) owing to the absence of characteristic allylicunsaturation on the polymer side chains, which prevented nanoparticlesynthesis. These results clearly support the hypothesis that theautoxidation process of unsaturated chains in drying oils is indeedresponsible for the reduction of metal salts.

After heating to approximately 60° C. for about 30 minutes, or uponexposure to the air (ambient conditions) for 10 hours, the sticky filmconverted into non-sticky (scratch-free) transparent insoluble film. Thecross-linking of PCA unsaturated hydrocarbon chains possibly occurredthrough the hydrogen insoluble polymer network (as shown in FIG. 1 b).This was thoroughly characterized using various techniques such asinfra-red spectroscopy, ¹H-NMR and differential scanning calorimeter(John 1993). Cross-linking may have occurred through hydroperoxidationof allylic radical centers of cardanol alkyl chain, and during thisprocess various free radicals were generated in situ (FIG. 1 b). In thecase of PCA, the side chains are brought close to each other by theacrylate polymer backbone and precisely aligned the allylic side chainsfor further cross-linking processes as shown schematically in FIG. 1 a.

Typically, metal salts were dissolved in a small amount of acetone, andadded to the chloroform solution of PCA. Subsequently, the polymer-metalsalts solution was coated on a glass slide. Evaporation of the solventproduced a sticky transparent colorless film. Upon exposure of thosefilms to the air at ambient conditions (i.e., a natural drying process)for 10 hours, the sticky films converted into non-sticky, scratch-freetransparent films. Visual changes were observed where the films turnedpink and yellow in color. The color change indicated the formation ofAuNPs and AgNPs, respectively, as shown in FIG. 2 a. Thesetransformations were also achieved by heating films at 60° C. for 30minutes. The in situ-generated free radicals, created during theautoxidation, are responsible for the reduction of metal salts togenerate MNPs. The stability and shelf life of NPs synthesized in thispolymer are comparable to the NPs synthesized using a conventionalprocess.

Example 2

In situ-prepared MNP-incorporated polymer films (e.g., alkyd resins)were characterized using different techniques includingultraviolet-visible spectrophotometry, transmission electron microscopy,scanning electron microscopy, energy-dispersive X-ray analysis, andX-ray photoelectron spectroscopy. The absorption spectrum ofnanoparticles generated in oil was monitored as a function of time, asshown in the inset of FIG. 5 e. It is clear from the spectra thatabsorbance at 450 nm increases as a function of time, and this peakappears for the AgNPs owing to the characteristic surface plasmonresonance effect originating from the quantum size of the AgNPs, whichagain confirms the formation of silver particles at nanoscale dimensions(Jin 2001). The absorbance maximum does not change over a long period,indicating that the silver particles are prevented from coagulatingowing to stabilization of nanoparticles by fatty acids, which areessential constituents of the drying oil. Similarly, absorption spectraof in situ-synthesized AuNP-containing oil have shown a surface plasmonresonance peak at 540 nm, characteristic of AuNPs.

In order to quantify the ratio of Ag⁺ to Ag⁰, X-ray photoemissionspectroscopy was performed. The Ag nanoparticles-oil medium is quitehomogeneous so chemical composition does not change from surface tobulk. A solution-cast film of silver nanoparticles in oil was formed onSi(111) substrates and analyzed by XPS. A general scan spectrum of thefilm at room temperature showed the prominently presence of C 1s, O 1s,and Ag 3d core levels with no evidence of impurities. The film wassufficiently thick and, therefore, no signal was measured from thesubstrate (Si 2p core level).

The Ag 3d core level spectra were recorded from the Ag-oil film formedby the drop-coating technique. The Ag 3d spectrum could be resolved intoa two spin-orbit pair (splitting ˜6 eV) with a 3d5/2 binding energy (BE)of 368.1 and 369.3 eV (the core levels were aligned with respect to theadventitious C 1s BE of 285 eV). This BE corresponds to that of Ag(0)and Ag(I) state of silver and are in good agreement with the reportedvalue. (Kumar 2003) The area occupied by peak is proportional to amountof silver present in different oxidation state in the samples. The ratioof area occupied of Ag(0) to Ag(I) peak is 7.5:1 indicate the relativeratio of metallic silver and silver ion present in the sample.

The absorption spectrums were recorded for metal salts that containedpolymer films before and after the drying process, as shown in FIG. 2 b.Absorption spectra of AuNPs that contained polymer showed acharacteristic surface Plasmon resonance band at 555 nm, whereas such apeak was absent when recording absorption spectra immediately afteraddition of metal salts (before the drying process). This suggests theformation of AuNPs during the cross-linking process, as shown in FIG. 2b: (i) and (ii). Similarly, absorption spectra of in situ synthesizedAgNPs in polymers show a peak at 460 nm. The peak is indicative of theAgNPs because of the characteristic surface Plasmon resonance effectoriginating from the quantum size effect of AgNPs (Jin 2001). In thisinstance, prior to the autoxidation (cross-linking) process, such anabsorption band in UV was absent, as shown in FIG. 2 b: (iii) and (iv).The formation of the absorption band suggests that MNPs are generatedduring the autoxidation process. The fact that the absorbance maximumdoes not change over a long period indicates that MNPs are preventedfrom coagulation because of stabilization of the NPs by the polymer.

FIG. 5 e shows a representative transmission electron micrograph ofAgNPs contained in the films (for more transmission electron micrographsof AgNPs and AuNPs. The average size of the AgNPs was found to be 12-14nm; however, larger sizes in the 10-30 nm range were also occasionallyobserved (FIG. 5 e). Similarly, the size range for AuNPs is 11-25 nmwith a higher polydispersity. The AgNPs were further characterized usingX-ray photoelectron spectroscopy.

FIG. 3 shows SEM and TEM images of in situ synthesized AuNPs and AgNPsin PCA. The synthesized MNPs were nearly monodispersed with the averagesize of 18 nm for AuNPs, and 13 nm for AgNPs. Polydispersity of theparticles were calculated from TEM data by plotting particle sizedistribution histogram

Additionally, energy-dispersive X-ray analysis was used to confirm thepresence of AgNP- and AuNP-embedded coatings. Thin films of AgNP- andAuNP-incorporated paints on silicon wafers were examined under ascanning electron microscope, where it was clear that the surfaces ofthe coatings were filled with metal nanoparticles. Spot analysis wascarried out using energy-dispersive X-ray spectroscopy on the areaswhere particles were located, in the range of 4 keV, and characteristicpeaks at 2.984 keV and 2.195 keV of silver and gold, respectively, wereobserved. In addition, the background materials showed representativepeaks for carbon and oxygen. These results clearly suggest the presenceof MNPs embedded in the drying oil-based paints.

Free radicals generated during cross-linking of unsaturated chains ofcardanol (autoxidation) are probably responsible for the reduction ofmetal salts to generate MNPs in situ. As a test, a saturated analogue,pentadecylphenyl acrylate (PDA) (shown in FIG. 1 c) was synthesized. PDSwas further polymerized similar to the cardanyl acrylate. The resultingpolymer, poly(pentadecylphenyl)acrylate (PPDA) has only saturatedchains. FIG. 4 shows chemical structures of PPDA, which is a saturatedanalogue of PCA. An acetone solution of metal salts was added to thePPDA in chloroform, and dropcasted on a glass slide akin to previousexperiments. Evaporation of solvent produced sticky transparent filmwhich remained as sticky transparent film even after seven days uponexposure to ambient conditions. The retention of the sticky characterevidences the absence of the cross-linking process. This suggests thatsaturated chains lack the ability to cross-link. After seven days, thesefilms were subjected to absorption spectroscopy. No surface plasmonresonance band corresponding to the MNPs appeared in the spectrum. Thisobservation suggests that indeed in situ generated free radicals duringcross-linking are responsible for the reduction of metal salt to produceMNPs in the polymer.

Free radicals generated during autoxidation are responsible for thereduction of metal salts. To prove this hypothesis, two sets ofexperiments were performed: one to enhance the reduction process byincreasing the free radical generation and the rate of AuNP formation,and the other to completely prevent metal reduction using free radicalscavengers.

It is well known that the addition of catalytic metals such as Co(II),Mn(III) and Fe(II) facilitates free radical formation and subsequentlyenhances the oxidative drying (lipid autoxidation) process (Van Gorkum2005, Tang 2000). Silver benzoate and chloroauric acid were reducedseparately by using oil in the presence of Fe(II) ions. The kinetics ofnanoparticle formation was observed to be enhanced threefold (the Ag⁺¹to Ag⁰ reduction time decreased from 360 to 120 min, see FIG. 5 f). Thekinetics of nanoparticle formation was studied using absorbance spectrameasurements of the in situ-synthesized AuNPs.

In the presence of a free radical formation-promoting catalyst, completeAgNPs formation was achieved in 2 hours. On the contrary, in the absenceof catalyst 6 hours were needed for completion. In both cases,absorbance did not change over a long period of time (12 hours),indicating the particle formation has been completed and NPs are stablein the oil media. Rate enhancement of nanoparticle formation in thepresence of a free radical initiator suggests that free radicals areindeed involved in the reduction process.

In the negative control experiments, the reduction process was carriedout in the presence of free radical scavengers. Dimethylsulphoxide(DMSO) is known to act as a free radical scavenger, and is frequentlyused to prevent free radical-mediated processes (Ahmed 1998). DMSO (25%v/v) was mixed with the oil, and metal salts were then added andincubated for several months. Intriguingly, there was no nanoparticleformation, which was confirmed by absorption spectroscopy. Hence, theseresults unambiguously show that free radicals are indeed mostlyresponsible for the reduction of metal salts in the drying process.

In situ-synthesized MNPs are stable for several months withoutcoagulation, owing to the passivation of MNPs with fatty acids andaldehydes. It is well documented that autoxidation ultimately leads toextensive fragmentation of the fatty-acid chains and generates a varietyof biologically active products such as monoaldehydes, γ-ketoaldehydes,and 4-hydroxy-nonenal through free radical intermediates (Yin 2004,Esterbauer 1991). In addition, acids, aldehydes and free radicals areknown to interact with MNPs to stabilize them (Vemula 2007, Nath 2004,Zhang 2003). Hence, the autoxidation reactions should produce fragmentsthat bind to in situ-generated MNPs. In addition, the rigid cross-linkedpolymer (the product of the drying process of oil) also preventsnanoparticle aggregation, resulting in the production ofnanoparticle-embedded homogeneous paints.

The AgNP-embedded drying oil is an excellent coating material and can beused to coat several kinds of surface such as wood, glass,polypropylene, poly(methyl methacrylate), polystyrene and building wallsmade of different materials. As MNPs are homogeneously dispersed invegetable and cashew nut-based drying oil, the adhesion properties ofAgNP- and AuNP-embedded paints were tested by coating them on differentsubstrates such as glass and polymers, as shown in FIG. 6. Toinvestigate the versatility of this process, several commerciallyavailable drying oil and paints were examined, such as bleached linseedoil, cold-pressed linseed oil, stand oil and Beckosol oils. In allcases, MNPs were successfully synthesized in situ by using the naturallyoccurring autoxidation as a tool during the drying process.

Silver is known to exhibit a broad spectrum of biocidal activity towardsmany bacteria, fungi and viruses (Russel 1994, Zachariadis 2004).Silver, in its uncharged state (i.e., an AgNP) is also found to possessantimicrobial properties. Although the mechanism of this action is stillunresolved, it has been shown that AgNPs interact with the constituentsof the outer membrane of bacteria, causing structural changes anddegradation that eventually lead to the death of the bacterial cells(Sondi, 2004). AgNPs that are less than 15 nm in size are known to haveefficient antibacterial activity (Morones 2005, Sudhir 1998). Hence,AgNP-embedded vegetable drying oils may be used as ‘antibacterialpaints’ on different surfaces.

The bactericidal activity of in situ-synthesized AgNP-embedded vegetabledrying oil was explored against both the airborne Gram-positive humanpathogen S. aureus and its Gram-negative brethren E. coli. Surfaces werecoated with the AgNP-embedded paint by either simple immersion of glassslides (2.5×7.5 cm) into the AgNP-embedded paint or spray coating ofAgNP-embedded paint on glass slides followed by drying under air, whichproduced uniformly coated films. Bacterial colonies were grown on theseslides, as discussed in the Methods section below.

Specifically, glass slides (2.5×7.5 cm) were coated with theAgNP-embedded paint by either simple immersion into AgNP-embedded paintor spray coating with the AgNP-embedded paint followed by drying in air.Both methods produced uniformly coated glass samples. The slides wereleft for one day or heated at 60° C. for 30 min to make sure that thedrying process was complete, so that a scratch-free film was formed.Typically, a 100 μL suspension of S. aureus or E. coli in 0.1 M PBS(approximately 10¹¹ cells ml⁻¹) was added to 20 ml of the yeast-dextrosebroth in a 50 mL sterile centrifuge tube, followed by shaking at 200 rpmand 37° C. overnight (Innova 4200 Incubator Shaker, New BrunswickScientific). The bacterial cells were collected by centrifugation at6,000 rpm for 10 min (Sorvall RC-5B, DuPont Instruments), washed twicewith PBS, and diluted to 5×10⁶ cells mL⁻¹ for S. aureus and to 5×10⁷cells ml⁻¹ for E. coli. To mimic the aerosolized, airborne bacteria, wesprayed the bacterial suspensions in PBS onto slides at a rate ofapproximately 10 mL min⁻¹ in a fume hood. After drying for 2 min in air,the resultant slide was placed in a Petri dish and immediately coveredwith a layer of solid growth agar (1.5% agar in the yeast-dextrosebroth, autoclaved, poured into a Petri dish, and allowed to gel at roomtemperature overnight). The Petri dish was sealed and incubated at 37°C. overnight, and the bacterial colonies grown on the slide surface werecounted on a light box. The control experiments investigated the effectsof a plain glass surface and paint without AgNPs; both types ofbacterial solution were sprayed separately on commercially availableglass slides and glass slides coated with oils without AgNPs,respectively.

The bacterial strains used were S. aureus and E. coli. Theyeast-dextrose broth contained (per liter of deionized water): 10 g ofpeptone, 8 g of beef extract, 5 g of NaCl, 5 g of glucose, and 3 g ofyeast extract. The PBS contained 8.2 g of NaCl and 1.2 g of NaH₂PO₄.H₂Oper liter of deionized water. The pH of the PBS solution was adjusted to7.0 with 1N aqueous NaOH. Both solutions were autoclaved for 20 minutesbefore use.

After overnight incubation of the slides, the bacterial colonies werecounted; images of the resulting glass slides are shown in FIG. 8. Eachblack dot corresponds to an individual bacterial colony. It can be seenin FIG. 8 a that the glass slide coated with AgNP-embedded paint killedalmost all of the Gram-positive S. aureus bacteria compared with boththe plain glass slide and the glass slide coated with paint withoutAgNPs.

In the present invention, the nanoparticles kill bacteria by rupturingtheir cell walls. As shown in FIG. 8, comparison of microbial growth oneach of a glass slide, a glass slide with paint, and a glass slide withpaint and nanoparticles, shows the antimicrobial effect of MNPs. Theantimicrobial activity was determined to be 100%, because no bacterialcolonies were observed under the microscope. The nanoparticles of thepresent invention also exhibit other significant antimicrobial activity,such as antiviral activity.

Similarly, AgNP-embedded vegetable paints were equally active againstthe Gram-negative E. coli bacteria (FIG. 8 b). In the case of E. coli,control experiments also showed that plain glass and glass coated withpaint without AgNPs failed to kill the bacteria, suggesting that AgNPsare indeed responsible for the bactericidal activity. Previously,Morones et al. showed that AgNPs (where silver is present in the Ag⁰form) also contain micromolar concentrations of Ag⁺ ions, and they haveshown that Ag⁺ and Ag⁰ both contribute to the antibacterial activity(Morones 2005). To quantify the ratio of Ag⁰ to Ag⁺, X-ray photoemissionspectroscopy was performed. The calculated ratio of Ag⁰ to Ag⁺ is 7.5:1.Hence, the silver ions and metallic silver both synergisticallycontribute to the enhanced antibacterial activity, in agreement with theearlier reports. The proficient bactericidal activity (see Table 1) ofAgNP-embedded vegetable oil paints against both types of bacteriumsuggests the use of the present AgNP-incorporated paint formulations inother antimicrobial (e.g., antibacterial) coatings. All antibacterialtests were carried out in triplicate and were done a minimum of twodifferent times to ensure reproducibility.

Table 1 shows bactericidal activity against airborne S. aureus and E.coli of glass slides coated with AgNPs embedded vegetable drying oil(AgNPs-paint). Bacterial suspensions (5×10⁶ cells/mL) in a PBS aqueoussolutions were sprayed onto a surface of interest, dried in air for min,placed in a Petri dish, covered with 1.5% solid growth agar, sealed, andincubated at 37° C. overnight, and then the colonies were counted.

TABLE 1 bactericidal efficiency, % Surface S. aureus E. coli blank glass0 0 glass + paint 0 0 glass + AgNPs-paint “nearly total” “nearly total”

For the “blank glass,” a commercially available glass slide (2.5×7.5 cm)was used as such without coating. For the “glass+paint,” a glass slidewas coated with vegetable drying oil (in this sample there are no AgNPspresent). For the “glass+AgNPs-paint” a glass slide was coated withAgNPs embedded vegetable drying oils (AgNPs-paint). “Bactericidalefficiency” is defined as the number of bacterial colonies/cm² observedfollowing cultivation on a coated slide divided by the number ofbacterial colonies/cm² observed on the corresponding non-coated slide,times 100%. All experiments were carried out at least in triplicate.

Example 3

FIG. 9 shows the absorbance intensity at 450 nm increases as a functionof time. This absorbance peak is due to the surface plasmon resonance ofsilver nanoparticles and is responsible for a characteristic yellowcolor. The plasmon absorbance intensity increase as a function of timeand becomes saturated after 6 days (FIG. 9A), indicating the completionof the reaction. FIG. 9B shows the UV-visible spectra of Ag-oil and pureoil film formed at glass substrate. The peak at 450 nm (curve 2) is dueto silver nanoparticles present in oil. However, pure oil (curve 1) doesnot show any unusual optical property. The color of the film does notchange over a long period of time, which indicates that Ag nanoparticlesare very stable and capped with oil molecules. This is an excellentcoating material which can be used to coat any kind of substrate,ranging from polar to nonpolar substrates using this silvernanoparticles containing oil.

FIG. 10 is a photograph of various materials such as pure oil coatedglass (a), Ag-oil coated glass (b), Polycarbonate (c),Poly(methylmethacrylate) (d). These coatings are quite stable andindicate the excellent adhesion property of Ag-oil on both hydrophilic(e.g., ceramics, glass) and hydrophobic (e.g., polymers, etc.)substrates.

Example 4

Drop-coated films of the Ag-alkyd resin were formed on carbon-coatedcopper grids by solvent evaporation technique for transmission electronmicroscopy (TEM) measurements. FIG. 11 shows representative TEM imagesof Ag-alkyd resin. It is clear from the TEM picture that the silvernanoparticles are discrete, quite uniform, and distributed all over thegrid (FIG. 11A). FIG. 11B shows the histogram of the particle sizedistribution measured from particles in FIG. 11A and other similarmicrographs. A Gaussian fit to the histogram yielded an average particlesize of 4.4±0.2 nm.

Example 5 Antimicrobial/Antibacterial Activity

Bacteria and Media. The bacterial strains employed were Staphylococcusaureus (ATCC 33807) and Escherichia coli (E. coli genetic stock center,CGSC4401). Yeast-dextrose broth contained (per liter of deionizedwater): 10 g of peptone, 8 g of beef extract, 5 g of NaCl, 5 g ofglucose, and 3 g of yeast extract. Phosphate-buffered saline (PBS)contained 8.2 g of NaCl and 1.2 g of NaH₂PO₄.H₂O per liter of deionizedwater. The pH of the PBS solution was adjusted to 7.0 with 1 N aqueousNaOH. Both solutions were autoclaved for 20 min prior to use.

Determination of Bactericidal Efficiency. A 100-μl suspension of S.aureus or E. coli in 0.1 M PBS (approximately 10¹¹ cells/mL) was addedto 20 ml of the yeast-dextrose broth in a 50-ml sterile centrifuge tube,followed by shaking at 200 rpm and 37° C. overnight (Innova 4200Incubator Shaker, New Brunswick Scientific). The bacterial cells wereharvested by centrifugation at 6,000 rpm for 10 min (Sorvall RC-5B,DuPont Instruments), washed twice with PBS, and diluted to 5×10⁶cells/ml for S. aureus and to 5×10⁷ cells/ml for E. coli. The bacterialsuspensions in PBS were sprayed onto slides at a rate of approximately10 ml/min in a fume hood. After a 2-min drying under air, the resultantslide was placed in a Petri dish and immediately covered with a layer ofsolid growth agar (1.5% agar in the yeast-dextrose broth, autoclaved,poured into a Petri dish, and allowed to gel at r.t. overnight). ThePetri dish was sealed and incubated at 37° C. overnight, and thebacterial colonies grown on the slide surface were counted on a lightbox.

The samples were tested against gram positive (Staphylococcus aureus)and gram negative (E. coli) bacteria. Triplicate was done for everycase. The plane glass slide was used as a control. 100% antibacterialefficiency means there was no colony in the entire slide after 24 hincubation. 0% antibacterial efficiency means there was no statisticallysignificant difference compared to the control slide as shown in Table2.

TABLE 2 Staphylococcus aureus E. coli Sample (gram positive) (gramnegative) Coated slide 100% 100% Uncoated slide  0%  0%

Example 6

The present invention relates to a simple shaking process either bycomplexing metal ion with drying oil molecules at the nonpolar solvent(paint thinner)-water interface or by dissolving a organometalliccomplex in oil medium and then in situ formation nanoparticles in theorganic phase. The critical goal of the present invention is to providean improved process for the synthesis of metal nanoparticles such asgold, silver, platinum, nickel, copper, ruthenium and palladium directlyin coating materials for various applications.

Another object is to make metal nanoparticles coated with hydrophobicgroups in various organic solvents, which can be used for commercialpurpose. Accordingly, the present invention provides a process for thepreparation of nanoparticles dispersion which comprises of mixing anaqueous solution of metal ion with a solution of drying oil in anorganic solvent under agitation (see FIG. 13).

Example 7

These nanoparticles have been characterized by various techniques suchas transmission electron microscopy (TEM), UV-visible spectroscopy etc.Observation of the TEM image showed that gold nanoparticles wereobtained using this protocol. The gold nanoparticles are discrete, quiteuniform and stable.

Average particle size was calculated by combining the data from the TEMimages The gold nanoparticles were with the size of 10 to 50 nm, majorportion of the nanoparticles are in the size dimension of ˜12 nm. Theseparticles are dispersed in vegetable oil paints therefore thin film ofthis metal nanoparticles dispersion can be obtained on any substrate. Todemonstrate the concept, the paint-containing nanoparticles were coatedonto various substrates such as ceramic, glass or silicon wafer, andpolymers. It was clear that coated glass and polymer look ruby red incolor and the coating was quite uniform, indicating that this dispersionadhere very well on these substrates.

This method could be equally and easily applied to the generation of artwork and designs in RUBY RED, in place of traditional golden yellow artwork in ceramic-ware and crockery industry. Dip coated film of goldnanoparticles-oil film was formed on glass and analyzed by UV-visiblespectrometer as a function of temperature. UV-Visible spectra were takenof film at room temperature and after heating at 200 and 400° C. for 1hr at each temperature. The absorbance at 560 nm corresponds to thesurface plasmon resonance, which was originally at 560 nm, shifts to thehigher wavelength region with temperature. This shift in surface plasmonof resonance is due to aggregation of the particles in the film. It islikely that above 200° C., desorption occur of polymerized oil from thegold nanoparticles and this leads to sintering of the particles.Aggregation of nanoparticles was also studied using atomic forcemicroscope. Dip coated film of gold nanoparticles-oil dispersion wasmade on silicon wafer and scan in tapping mode using Nanoscope IIIinstrument.

The AFM image was obtained for the dip coated film of gold oildispersion on silicon wafer. The line profile was also obtained. The AFMimage was obtained for the same film after heating at 400° C. for onehour and corresponding line profile. It is clear from the line profilethat gold film is quite uniform and individual gold nanoparticles arenot seen in the image. It indicates that particles are covered withpolymers form due to oxidation of oil. On annealing the film become veryrough due to the aggregation of gold nanoparticles and individualaggregates can be seen in AFM image which is in good agreement with theUV-visible studies.

This nanoparticle synthesis process was also simplified andorganometallic compounds (e.g., silver benzoate) were used as aprecursor for the synthesis of metal nanoparticles directly in oil. Inthis process, organometallic compounds were dissolved directly in oilphase, which got decomposed during drying process of oil and formnanoparticles.

Example 8

This example illustrates the synthesis of gold nanoparticles with dryingoil at the hexanewater interface and then the phase transfer of goldnanoparticles in the organic phase. In a typical experiment, 30 mL of a10⁻² M solution of gold chloride, (Sigma chemicals, used as-received) inwater was added to 30 mL hexane containing 3 gm of oil (Miniwax, Antiqueoil finish, used as-received). This biphasic solution was kept in darkfor 24 hour under agitation for the formation of nanoparticles inorganic phase.

Example 9

This example illustrates the synthesis of silver nanoparticles withdrying oil at the hexane-water interface and then the phase transfer ofsilver nano-particles in the organic phase. In a typical experiment, 30mL of a 10⁻² M solution of silver nitrate, (Sigma chemicals, usedas-received) in water was added to 30 mL hexane containing 3 gm of oil.(Miniwax, Antique oil finish, used as received). This biphasic solutionwas kept in dark for 24 hour under agitation for the formation ofnanoparticles in organic phase.

Example 10

This example illustrates the synthesis of copper nanoparticles withdrying oil at the hexane-water interface and then the phase transfer ofcopper nanoparticles in the organic phase. In a typical experiment, 30mL of a 10⁻² M solution of copper sulfate, (Sigma chemicals, usedas-received) in water was added to 30 mL hexane containing 3 gm of oil(Miniwax, Antique oil finish, used as-received). This biphasic solutionwas kept in dark for 24 hour under agitation for the formation ofnanoparticles in organic phase.

Example 11

This example illustrates the synthesis of silver nanoparticles withdrying oil. In a typical experiment 0.034 gm of silver benzoate wasdissolved in 75 mL of toluene. 4.8 gm of alkyd resin was mixed in thesolution and kept it in dark for 12 hours. The oil become yellow after12 hours due to formation of silver nanoparticles.

Methods

Nanoparticle Synthesis In Oil. In one experiment, 0.034 g silverbenzoate was dissolved in 4.8 g alkyd paint (all experiments were donewith Miniwax, Antique oil finish unless otherwise specified, which wasused as-received) and was mixed to form a homogeneous solution and keptat room temperature. Similarly, for the synthesis of AuNPs, 0.136 g ofHAuCl₄ was added to 20 mL of drying oil and was mixed with a glass rodto form a green colored oil, which remained as a gold salt for threeweeks. For the MNP synthesis, oil containing metal salts was coated onthe glass surfaces and left to dry in air, which caused autoxidationthat subsequently led to the formation of MNP-embedded drying-oilscratch-free coatings.

Poly(Cardanyl Acrylate) Synthesis. Cardanol was obtained by doublevacuum distillation of cashew nut shell liquid at 3-4 mm Hg; thefraction distilled at 230-235° C. was collected. The monomer, cardanylacrylate and poly(cardanyl acrylate) were synthesized as reportedearlier (John 1992, John 1993).

Nanoparticle Synthesis In Synthetic Polymer. Metal salts (silverbenzoate or chloroauric acid) were dissolved in acetone, and added to achloroform solution of poly(cardanyl acrylate). The mixture was coatedon the glass slides and left in air to dry, which caused autoxidationand the subsequent formation of MNP-embedded polymeric scratch-freecoatings.

Transmission Electron Microscopy. Transmission electron microscopy datawere recorded using a Zeiss EM-902 transmission electron microscope (80kV). Nanoparticle-embedded oil was placed directly on a 10×10 mm plasticsheet, and after drying at 60° C. for 4 h, it was cut using a microtometo obtain 100-nm-thick slices, which were placed directly on a Cu gridand examined under a transmission electron microscope.

Scanning Electron Microscopy And Energy-Dispersive X-Ray Spectroscopy. Asmall amount of nanoparticle-embedded oil was placed on a silicon waferto form a thin layer; the silicon wafer was dried at ambient conditionsfor 24 h, and was directly examined using a field-emission scanningelectron microscope (JEOL-6330F) operated at 5 kV. Energy-dispersiveX-ray data were also collected from the same sample at 15 kV using aPrism 2000 Si(Li) X-Ray detector (Princeton Gamma-Tech) coupled with aZeiss DSM-940 microscope.

Ultraviolet-Visible And X-Ray Photoelectron Spectroscopy.Nanoparticle-embedded oil was placed on glass slides and dried to formthin films, which were examined directly with a Perkin Elmer Lamda-950spectrophotometer operated at a resolution of 2 nm and a PHI-5400instrument with a 200 W Mg Kα probe beam. The spectrometer wasconfigured to operate at high resolution with a pass energy of 20 eV.

Characterization of poly(cardanyl acrylate). The number- andweight-average molecular weights of polymer was determined by a WatersAssociates model 440 gel permeation chromatograph having μ-styragelcolumn of pore size 10⁵, 10⁴, and 10³ Å connected in series and UVdetector. Chloroform was used as the mobile phase. The instrument wascalibrated using standard samples of polystyrene. The number- and weightaverage molecular weights of the polymer before cross-linking asdetermined by GPC were 3000 and 11,000 respectively.

DSC: DSC was recorded using a Mettler TA 3000 system in air at a heatingrate of 5° C./min from 30 to 200° C. DSC curve of poly(cardanylacrylate) shows an exotherm at 80-185° C. and a peak maxima of 132.1°C., indicating a polymerization reaction. This might be due tocross-linking of polymer.

IR: The C—H stretching vibrations of the unsaturated part of the sidechain of the monomer CA was observed at 3020 cm⁻¹, which ischaracteristic of disubstituted olefinic system. This is furtherclarified from the observation that this peak is absent in saturatedanalog of cardanyl acrylate where the side chain has no double bond. Thedouble bond of acrylic moiety was observed at 1658 cm⁻¹, and in thepoly(cardanyl acrylate), the peat at 1658 cm⁻¹ was not observed, due tothe conversion of acrylic C═C to C—C, where as the peak at 3020 cm⁻¹ didnot change. This is clearly indicating that the side chain of monomerwith its double bonds remains intact in the polymer and polymerizationtakes place only through the acrylate double bonds.

¹H-NMR: In the ¹H-NMR of cardanyl acrylate monomer the peaks wereassigned for different protons (δ, ppm): 0.8 (t, CH₃), 1.11-1.5 [m,(CH₂)], 1.7-2.2 (nr, —CH₂—CH═CH—), 2.2-2.8 (nr, —CH₂—Ar), 5.2-5.5 (t,CH₂═CH—, —CH═CH—), 5.7-6.1 (m, ═CH₂), 6.3-6.6 (m, —CH═), 6.8-7.3 (m,aromatic). In the NMR spectrum of poly(cardanyl acrylate) the peak atδ=5.7-6.6 (acrylic) is not present due to polymerization throughacrylate moiety and the peak at δ=5.2-5.5 for the double bonds in theside chain of the monomer remains intact after the polymerizationreaction, which also supports the IR data.

In summary, a renewable resources based monomer (cardanol from CNSL) wasused to prepare a biobased polymer. In situ synthesis of MNPs has beenachieved with this polymer without using any external reducing andstabilizing agents. In situ-generated free radicals during oxidativedrying of cardanol unsaturated chains were used as a tool for MNPssynthesis, and was confirmed by synthesizing saturated analogue polymerwhich failed to generate MNPs. MNP-embedded polymers films were stableat ambient conditions for a longer period. The preparation oforganic-inorganic hybrid materials may have applications in developingmaterials with tunable optical, electrical, and catalytic properties.AgNPs embedded biobased polymers have potential antibacterial activity.

BIBLIOGRAPHY

-   Abyaneh, et al., J. Phys. D: Appl. Phys. 2007, 40, 3771-3779-   Ahmed-Choudhury, et al., Toxicol. Appl. Pharmacol. 152, 270-275    (1998)-   Alt, et al., Biomaterials 25, 4383 (2004)-   Anastas & Williamson, Green Chemistry: Frontiers in Benign Chemical    Syntheses and Processes (Oxford Univ. Press, Oxford, 1998)-   Aymonier, et al., Chem. Commun. 3018-3019 (2002)-   Baschong, et al. J. Electron Microsc. Tech., 1990, 14, 313.-   Berger, et al., Antimicrob. Agents Chemother. 9, 357-358 (1976)-   Bieleman, Additives for Coatings (Wiley-VCH, Weinheim, 2000)-   Biermann, et al., Angew. Chem. Int. Ed. 2000, 39, 2206-2224-   Bishop, et al., Gold Bull. 35 (3), 89, 2002.-   Black, J. Am. Chem. Soc. 100, 527-535 (1978)-   Bohannon, Science 309, 376-377 (2005)-   Bond, et al. Catal. Rev. Sci. Eng. 41, 319 (1999)-   Brust, et al. J. Chemn. Soc. Commun., 1994, 801-   Chou, et al. Polym. Adv. Technol. 16, 600-607 (2005)-   Colvin, et al., Nature 1994, 370, 354-   Corma, et al., Chem. Rev. 2007, 107, 2411-2502-   Crisp, et al., Nano Lett. 3, 173-177 (2003)-   Dai, et al., Nano Lett. 2, 497-501 (2002)-   Daniel, et al., Chem. Rev. 2004, 104, 293-346-   Daniel (ed.), Bailey's Industrial Oil and Fat Products (Wiley, NY,    1964)-   Dowling, et al. Surf Coat. Technol. 163, 637-640 (2003)-   Elghanian, et al., Science, 1997, 277, 1078-   Esterbauer, et al., Free Radic. Biol. Med. 11, 81-128 (1991)-   Fustin, et al., Langmuir 2006, 22, 6690-6695-   Gogoi, et al., Langmuir 22, 9322-9328 (2006)-   Goldemberg, Science 2007, 315, 808-810-   Haldar, et al., Proc. Natl. Acad. Sci. USA 103, 17667-17671 (2006)-   Heilmann, A. Polymer Films with Embedded Metal Nanoparticles    (Springer, NY, 2002)-   Hoffman, et al. J. Phys. Chem., 1992, 96, 5546.-   Hotchkiss, et al., Chem. Mater. 2007, 19, 6-13-   Iwakoshi, et al., Proceedings of the International Conference    ‘Gold2003: New Industrial Applications for Gold’, Vancouver, Canada,    Sep. 28-Oct. 1, 2003-   Jiang, et al., J. Appl. Polym. Sci. 93, 1411-1422 (2004)-   Jin, et al., Science 2001, 294, 1901-1903-   John, et al., Makromol. Chem., Rapid. Commun. 1992, 13, 255-   John, et al., J. Polym. Sci. Part A: Polym. Chem. 1993, 31,    1069-1073-   John, et al., Adv. Mater. 2001, 13, 715-718-   John, et al., Chem. Eur. J. 2002, 8, 5494-5500-   John, et al., J. Am. Chem. Soc. 2004, 126, 15012-15013-   John, et al., Angew. Chem. Int. Ed. 2006, 45, 4772-4776-   John, et al., Soft Matter 2006, 2, 909-914-   Kenawy, et al., Biomacromolecules 8, 1359-1384 (2007)-   Klabunde, et al. Nanoscale materials in chemistry(2001)-   Klaus, et al., Proc. Natl. Acad. Sci. USA 96, 13611-13614 (1999)-   Kumar, A. et al. J. Colloid Interface Sci. 264, 396-401 (2003)-   Lansdown, J. Wound Care. 11, 125-130 (2002)-   Lewis, et al., Trends Biotechnol. 23, 343-348 (2005)-   Lichtenthaler, Acc. Chem. Res. 2002, 35, 728-737-   Liu, et al., Anal. Chem. 2007, 79, 2221-2229-   Lu, et al., Lett. 5, 5-9 (2005)-   Mallia, et al., Angew. Chem. Int. Ed. 2007, 46, 3269-3274-   Metzger, et al., Appl. Microbiol. Biotechnol. 71, 13-22 (2006)-   Morones, et al., Nanotechnology 16, 2346-2353 (2005)-   Morones, et al., Langmuir 2007, 23, 8180-8186-   Naik, et al., Nature Mater. 1, 169-172 (2002)-   Nath, et al., Ind. J. Chem. A 43, 1147-1151 (2004)-   Okitsu, et al., Maeda, Chem. Mater. 1996, 8, 315-317-   Okitsu, et al., J. Phys. Chem. B 1997, 101, 5470-5472-   Pagliaro, et al., Angew. Chem. Int. Ed. 2007, 46, 4434-4440-   Podsiadlo, et al. Langmuir 21, 11915-11921 (2005)-   Prashant, et al. Biotech & Bioeng (2005) 90, 59-   Raveendran, et al., J. Am. Chem. Soc. 125, 13940-13941 (2003)-   Reich & Stivala, Autoxidation of Hydrocarbons and Polyolefins    (Marcel Dekker, NY, 1969)-   Rostrup-Nielsen, Science 2005, 308, 1421-1422-   Russel, et al., Prog. Med. Chem. 31, 351-370 (1994)-   Sambhy, et al., J. Am. Chem. Soc. 128, 9798-9808 (2006)-   Shan, et al., Macromolecules 2003, 36, 4526-4533-   Shan, et al., Macromolecules 2005, 38, 2918-2926-   Shankar, et al. Chem. Mater. 2005, 17, 566.-   Shimmin, et al., Langmuir 2004, 20, 5613-5620-   Shipway, et al. Chem. Mater. 1999, 11, 13-15-   Sondi, et al., J. Colloid Interface Sci. 275, 177-182 (2004)-   Sudhir, Langmuir 14, 1021-1025 (1998)-   Tang, et al., Biochem. J. 352, 27-36 (2000)-   Taton, et al. Science 2000, 289, 1757.-   Tyman, Chem. Soc. Rev. 1979, 8, 499-537-   Van Gorkum, et al., Coordination Chem. Rev. 249, 1709-1728 (2005)-   Vemula, et al., Chem. Commun. 2006, 2218-2220-   Vemula, et al., J. Am. Chem. Soc. 2006, 128, 8932-8938-   Vemula, et al., Chem. Mater. 2007, 19, 138-140-   Wang, et al. Chem. Mater. 2006, 18, 1988-1994-   Williams, et al., Crit. Rev. Biocompat. 5, 221-243 (1989)-   Yin, et al., J. Biol. Chem. 279, 3766-3776 (2004)-   Yin, et al., Antioxid. Redox Signal. 7, 170-184 (2005)-   Zachariadis, et al., Eur. J. Inorg. Chem. 2004, 1420-1426 (2004)-   Zhang, et al., J. Am. Chem. Soc. 125, 7959-7963 (2003)-   Zhang, et al., Angew. Chem. Int. Ed. 2006, 45, 1116-1119-   Zheng, et al., Langmuir 2004, 20, 4226-4235

All references cited and/or discussed in this specification areincorporated herein by reference in their entireties and to the sameextent as if each reference was individually incorporated by reference.

1. An antimicrobial composition, comprising a homogeneous mixture of adrying oil and metallic nanoparticles having a particle size of 1 to 50nm, wherein the composition is effective as an antimicrobial.
 2. Theantimicrobial composition of claim 1, wherein the drying oil is selectedfrom cashew nut shell liquid, linoleic acid, poppy oil, soyabean oil,urushi oil, linseed oil, sunflower oil, tung oil, alkyd resins, andcombinations thereof.
 3. The antimicrobial composition of claim 1,wherein metallic nanoparticles comprise a metal selected from silver,gold, nickel, platinum, palladium, cadmium, zinc, copper, andcombinations thereof.
 4. The antimicrobial composition of claim 1,wherein the metallic nanoparticles are provided on at least a portion ofthe surface of the composition and are present in an amount that isantimicrobially effective and that is less than a cytotoxic silverconcentration.
 5. The antimicrobial composition of claim 1, wherein themetallic nanoparticles are present in an amount ranging from more than 1nmol/L to less than 1 μmol/L.
 6. The antimicrobial composition of claim1, wherein the metallic nanoparticles are dispersed in the drying oil.7. The antimicrobial composition of claim 1, wherein the metallicnanoparticles are present in an amount of 1 to 2,000 ppm.
 8. Theantimicrobial composition of claim 7, wherein the metallic nanoparticlesare present in an amount of 5 to 1,000 ppm.
 9. The antimicrobialcomposition of claim 8, wherein the metallic nanoparticles are presentin an amount of 10 to 250 ppm.
 10. A method for preparing metalnanoparticles in a drying oil, comprising the steps of: (a) mixing asolution comprising metal ions with a solution comprising a drying oilin the presence of an organic solvent or an organometallic compound; (b)agitating the mixture for a period of 12 to 24 hours; and (c)polymerizing the drying oil by autoxidation to form metal nanoparticlesin a polymerized oil.
 11. The method of claim 10, wherein the metalnanoparticles are hydrophobic.
 12. The method of claim 10, wherein themetal ions are selected from gold, silver, nickel, platinum, palladium,cadmium, zinc, copper, and combinations thereof.
 13. The method of claim10, wherein the drying oil is selected from cashew nut shell liquid,linoleic acid, poppy oil, soyabean oil, urushi oil, linseed oil,sunflower oil, tung oil, alkyd resins, and combinations thereof.
 14. Themethod of claim 10, wherein step (a) occurs in the presence of anorganic solvent selected from n-hexane, chloroform, heptane, octane,petroleum ether, benzene, toluene, turpentine, and combinations thereof.15. The method of claim 10, wherein step (a) occurs in the presence ofan organometallic compound selected from silver benzoate, metal acetylacetonates, metal carbonyls, nonpolar metal salts, and combinationsthereof.
 16. The method of claim 15, wherein the organometallic compoundis selected from iron acetyl acetonate, platinum acetyl acetonate,nickel acetyl acetonate, cobalt acetyl acetonate, cobalt acetate, ironpetacarbonyl, and combinations thereof.